This invention relates to methods and apparatus for handling large flat and generally very thin flexible objects, and specifically to methods and apparatus for transporting, supporting, positioning, and constraining, with high mechanical precision, large flat flexible media. More specifically, this invention relates to the use of such transport and constraint mechanisms and techniques for automated optical inspection (AOI), electrical functional inspection (e.g., Voltage Imaging or VI) or automated repair (AR) of large flat, flexible and possibly patterned media, such as glass panels deposited with structures used to form thin film transistor (TFT) arrays (which are the main active component of liquid crystal flat panel displays (LCD). Although the invention is applicable to the general case of inspection of any flat flexible media, it is particularly useful for the high throughput, in-line inspection of glass plates of TFT/LCD panels at various stages of production.
During the manufacturing of LCD panels, large clear sheets of thin glass are used as a substrate for the deposition of various layers of materials to form electronic circuits that are intended to function as a plurality of separable, identical display panels. This deposition is usually done in stages where in some stages, a particular material (such as metal, Indium Tin Oxide (ITO), Silicon, Amorphous Silicon etc.) is deposited over a previous layer (or upon the bare glass substrate) in adherence to a predetermined pattern. Each stage may also include various other steps such as deposition, masking, etching, and stripping.
During each of these stages, and at various steps within a stage, many production defects may occur, that have electronic and/or visual implications on the final performance of the LCD product. Such defects include but, are not limited to: circuit shorts, opens, foreign particles, miss-deposition, feature size problems, over and under etching. The most common defects, shown in FIG. 1, include: metal protrusion 110 into ITO 112, ITO protrusion 114 into metal 116, a so-called mouse bite 118, an open circuit 120, a short 122 in a transistor 124, and a foreign particle 126.
In the preferred application domain such as the inspection and repair of TFT LCD panels, the defects subject to detection and repair can be as small as several microns in size, placing demanding defect detection limits on inspection and repair systems. Moreover, mere detection of defects is insufficient. Detected defects must also be classified as process defects, i.e. minor imperfections which do not undermine the performance of the finished product but are an early indication of the array manufacturing process drifting out of optimum conditions; reparable defects, which can be repaired, thus improving the array production yield; and finally killer defects, which disqualify the TFT array from further use.
Achieving this level of detection and classification often requires a two stage imaging process. An initial comparatively low resolution imaging process is used in a fast detection mode to detect a number of points of interest—POI (or defect candidates) over the entire surface inspected. A second comparatively high resolution imaging process is used to review and further image these POIs as part of a high resolution image analysis and classification process. Such systems require a very high degree of mechanical precision as will be explained below in relation to FIGS. 2 and 3.
FIG. 2 illustrates the six degrees of freedom for any object in motion in three-dimensional space: namely, linear motion along the three orthogonal axes as well as rotation around any of these axes. This framework is valid for all moving elements in a typical surface inspection system. Motion along each of these degrees of freedom may be intentional (due to actuation) or unintentional (due to mechanical inaccuracy in the system). For example, as an object is linearly translated along the y-axis, there may be a uncontrolled roll around the y-axis, a yaw around the z-axis, and a pitch around the x-axis. Usually, a mechanical stage translates or rotates an object along selected degrees of freedom while attempting to constrain the object from translating or rotating along the remaining ones. However, due to the inability to achieve perfect mechanical control, the uncontrolled movements along any of these remaining degrees of freedom lead to the system exhibiting a reduced mechanical precision. The mechanical precision of such a system can often be characterized by the accuracy, the repeatability, and the resolution. Accuracy measures how closely a mechanical positioning system can approach the instructed target position in the steady state. The repeatability on the other hand, measures how close the final steady state positions are to each other on repeated attempts to move to the same target position, possibly from different initial positions. The resolution is defined as the smallest incremental motion possible along a given degree of freedom.
FIGS. 3A and 3B illustrate a simplified example inspection system for large area flat media, which is one focus of the present invention. The system may be transformed into a repair instrument by changing the payload on the illustrated gantry 316. In this particular configuration, a low and high-resolution optical inspection task is explained. In a typical system, there are multiple low-resolution inspection cameras (typically each with 3.0–15.0 μm/pixel object plane resolution) that are part of a low resolution system 312 and one or more high resolution inspection cameras (typically each with 0.5–1.0 μm/pixel object plane resolution) that are part of a high resolution system 310.
Flat media 318 under inspection is transported over a precision surface 320 approximating a plane with tight flatness specifications. For example, ±2.0 μm z-axis variation over 1 m is achievable. The low resolution imaging system 312 and the high resolution imaging system 310 are mounted by means of precision gantry 316 over the surface. The mechanical stage is designed so that either of the imaging systems can be used to image any arbitrary point on the media surface 318. Furthermore, the imaging system requirements, such as focal length and depth of field dictate that the distance from the imaging system to the surface is controlled during the imaging process to within 1.0 μm to assure that the depth of field limitation of ±1.5 μm is not violated. There are multiple means of achieving this positional control. For example, one can let both imaging modules remain stationary in the x-axis and y-axis and move the media to be inspected 318 over the surface 320 while having z-axis actuation on the imaging modules to control focus. An alternative is to have only y-axis motion on the media to be inspected while incorporating x-axis and z-axis actuation into the imaging modules. Still another alternative is to have the media to be inspected completely stationary while having a moving gantry 316 over the surface 320. Note that each of these configurations will shift the precision requirements onto another part of the stage, will impact the size of the stage and will also result in a particular distribution of mechanical complexity within the system.
To illustrate how mechanical precision affects the system operation, assume that the system operation consists of the x-axis and y-axis scanning motion 322 of the media to be inspected over the surface 320. Also assume the typical configuration of a line scan low resolution imaging module and an area scan high resolution imaging module. In such an inspection system, the following requirements on mechanical precision are present:
The field-of-view (FOV) of both the low resolution and the high resolution imaging modules, combined with the need to cover the entire surface of the media in multiple passes, necessitates high resolution for the x-axis position control and very high rotational stiffness around the z-axis. For example, 0.5 μm/pixel high resolution imaging using a particular line-scan camera would result in 0.4 mm x-axis FOV. This in turn would require a defect point of interest to be positioned with better than ±0.1 mm positional accuracy to within the camera FOV. The time-domain-integration (TDI) line scan imaging devices often used in low illumination intensity applications also require a consistent y-axis scanning speed to prevent image blurring. For example, a 96 stage TDI camera for a fixed integration time would suffer from one pixel image blurring from approximately 1% speed variation along the direction of scan
The limited depth of field of the imaging systems, in particular for the high resolution imaging module, requires that the distance from the inspected surface to the imaging module be tightly controlled. This distance is, for example, ±1.5 μm for a typical high resolution system with 0.5 μm object plane resolution. This requires tight accuracy and repeatability in z-axis positioning and high rotational stiffness around the x and y-axes.
In order to dispatch the high resolution imaging module to the POIs indicated by the low resolution imaging module, high accuracy and repeatability is required for the x-axis and y-axis motion. Also, there should be a known stable positional relationship between the low and high resolution modules.
In practice, apart from the aforementioned positional accuracy and repeatability requirements, there may be more complex relationships involved. For example, in an optical imaging system, any misalignment of the optical axis from vertical may cause a z-axis positional change to affect the x-axis and y-axis positioning accuracy of the field of view of the imaging module.
When an application requires a high mechanical precision, the widely adopted method of providing this precision is to use a massive granite base plate and associated stiff gantry (often from granite) supporting a rigid chuck. Over the reference flat surface provided by the granite, the chuck is levitated on air bearings and is actuated by means of linear servo motors and linear encoders. The chuck usually uses vacuum as the means to constrain the media being processed to the chuck surface. This approach has been especially used for the inspection of silicon wafer integrated circuits and has also been adopted for the inspection and repair of the glass plates deposited with TFT/LCD panels.
In this configuration, the precision machined granite base plate and stiff gantry provide a precision reference frame with high stiffness and flatness. The vacuum chuck holds the flexible media to be inspected and imposes the required flatness constraints. The chuck performs a precisely controlled motion over the granite support surface. Air bearings are the best known means of constraining free movement into a single axis. They provide an inherent averaging property due to the fact that the moving shuttle does not exactly follow the imperfections of the supporting guide but on the air cushion, which produces averaging. This results in much lower linear and angular errors for the shuttle as compared to the errors implied by those supporting surfaces. The linear servo motors in combination with linear encoders provide the necessary motion precision along the actuated motion axis.
This x-y-z stage configuration employing a granite base plate, vacuum chucks, air bearings, and linear encoders is a stable platform and is adequate for numerous applications. It has been successfully used in AOI and in electrical functional inspection of silicon wafer integrated circuits, which is believed to be the most demanding application domain. Although the concept has also been extended to the AOI and electrical functional inspection of the glass plates deposited by TFT/LCD panels, limitations in this particular domain have been the weight and size. The maximum feasible size achievable by this configuration is primarily limited by the weight and size of the required monolithic granite base plate which can be feasibly manufactured, stored, transported and installed.
In the primary application domain of interest, the inspection of TFT/LCD glass plates, the size of the glass plates is constantly increased as the industry strives for larger and thinner glass. With the increased size of the media to be inspected, the needed size of the stage to transport, position, and constrain the media grows proportionally. For Generation 5 (˜1,100 mm×1,300 mm glass) plate sizes, the direct scaling of the aforementioned configuration gradually ceases to be feasible. This is due to the weight, shape, and size of the instrument, which exceed the typical truck and plane cargo space capacity. (e.g., the maximum allowance for the bulk load of a commercial cargo plane is approximately seven tons while the for Generation 6 (˜1,500 mm×1,850 mm glass) plate sizes, the weight of the stage is predicted to be 11 tons.) The result is an exponential increase in the cost of transporting the instrument to its final destination.
In the past, the conventional method of providing the necessary mechanical precision was based on techniques in the silicon wafer integrated circuit inspection application domain. However, with the increasing size of the media panels to be inspected, this approach quickly becomes impractical due to the unmanageable size of the stage and the escalating cost that arises.
In the prior art, there are many applications where conveyor systems to transport and constrain media are proposed for the purposes of inspection or other processing of flat media. These include but are not limited to:
U.S. Pat. No. 6,367,609 and U.S. Pat. No. 6,223,880 both to Caspi et al. describe a conveyor system with the aim of changing the direction of media to be processed to divert it into an inspection or processing apparatus where the media is constrained using a vacuum chuck or similar means. The patents address the issue of transporting and handling of flat media on a production line for the purposes of processing or inspection. However, the patent does not address the required complexity and precision requirements and the associated cost implications of the inspection/processing station. This is one of the primary objects of the present invention. Also, the described conveyor apparatus uses primarily belt driven actuation for transporting the media.
U.S. Pat. No. 4,730,526 to Pearl et al. describes a conveyor system for supporting and transporting sheet media for the purposes of processing of the sheet media. The invention discloses a vacuum constraining mechanism with distributed vacuum pads distributed among the conveyor so that vacuum constraint happens together with the transportation and possible processing of the sheet media. The invention is especially useful for tooling applications such as cutting and is not applicable to the present application domain because of differing precision requirements.
U.S. Pat. No. 6,145,648 to Teichman et al. describes a conveyor arrangement for the purposes of PCB inspection, where a continuous conveyor extends from a loader zone to an unloader zone and passes by an inspection zone for the purposes of inspecting the articles traveling on the conveyor. The primary feature of the described invention is to operate the loader and unloader robots in a coordinated way, avoiding disturbance of the inspection process when the article is being inspected by the inspection apparatus.
U.S. Pat. No. 6,486,927 to Kim describes an LCD module testing apparatus with an index feeding stage for transferring the LCD modules from a LCD stack to a work table mounted on a main frame of the testing apparatus. The testing system is based on aligning the LCD module, placing it on electrical probe pins, and constraining it there mechanically for performing the test. The system does not attempt to handle, test, and repair the large size media sheets on which the LCD panels are deposited and hence is not applicable to the application of the present invention.
U.S. Pat. No. 5,374,021 to Kleinman describes a vacuum holder to be particularly used in a vacuum table arrangement. The invention specifically addresses the issue that when the vacuum table area is large and a major area is not covered by the article being held by the vacuum, suction openings cause the waste of vacuum. The invention proposes vacuum openings with a valve structure, which closes when no article is present on top of the valve.
U.S. Pat. No. 5,141,212 to Beeding describes another vacuum chuck concept which uses a foam surface to support sheet media during cutting operations. The open cell foam passes the effect of vacuum from the underlying vacuum surface to the media being held and is cut by the cutting apparatus along with the media. The underlying vacuum surface is therefore kept intact during this operation.
U.S. Pat. No. 5,797,317 to Lahat et al. describes a universal chuck concept for holding plates of varying sizes. The invention uses a means to mechanically hold the plates from the edges and primarily applies to small sized plates (e.g. silicon wafers), such as those typically used in the manufacturing of semiconductor devices.
U.S. Pat. No. 5,056,765 to Brandstater describes a means to constrain the flat media being processed or inspected by the use of an immobilizing device acting from the top of the media, which presses the media down without contact using an air-cushion effect. The media is hence flattened against the inspection surface by the immobilizing device, which is still free to move with respect to the flat media and the table. The invention in particular applicable for printed circuit inspection.
Contributions from the other application domains such as paper copiers include the U.S. Pat. No. 6,442,369 to Swartz et al., which describes an air cushion means of loading the media sheets from the top. The load imposes non-contact z-axis flatness on the sheets while the sheets are pressed against a conveyor for transportation. The sheets are constrained and moved by the underlying conveyor while being free to move with respect to the air cushion load.
Another earlier invention, U.S. Pat. No. 5,016,363 to Krieger, describes a vacuum and air cushion arrangement for transporting and at the same time drying a wet continuous web of media, in particular paper. However, no attempt is made to constrain the flatness of the conveyed media.
In U.S. Pat. No. 5,913,268, Jackson et al. describe pneumatic rollers, which utilize alternating vacuum and air cushion operation to gracefully transport and transfer sheet paper media between the rollers of a processing instrument, in particular for the purposes of printing on the media.
Despite these contributions in related application domains, the primary approach to designing a high precision mechanical stage remains the monolithic granite approach. This popular approach has been in the public domain and shared by a number of manufacturers of inspection/repair systems for both silicon wafer integrated circuits as well as for glass plates deposited with TFT/LCD panels.